1. Field of the Invention
The present invention relates to an imaging apparatus and an in-vivo image obtaining apparatus such as a capsule endoscope using the imaging apparatus.
2. Description of the Related Art
In recent years, a swallowable capsule endoscope which is configured by storing, in a case having a capsule shape, an imaging apparatus provided with an imaging unit that obtains image information of an inside of a subject, an illumination unit that illuminates a region to be imaged by the imaging unit, a transmitting unit that wirelessly transmits the image information obtained by the imaging unit, and the like, has been proposed and come to a stage of practical use in the field of endoscopes (see Japanese Patent Application Laid-Open No. 2003-19111, for example). Such a capsule endoscope is swallowed from a mouth of a patient as a subject and inserted into an inside of the subject. Then, the capsule endoscope, while traveling inside a body cavity according to the peristalsis, sequentially captures images of the inside of the body cavity at an interval of 0.5 second, for example, during a period until it is naturally excreted and wirelessly transmits obtained image data to a receiver placed outside the subject.
FIG. 1 is a block diagram showing an example of a schematic structure of an imaging apparatus housed in a conventional capsule endoscope. As shown in FIG. 1, the conventional imaging apparatus is constituted by: an image sensor 201 which is formed by an imaging device such as a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor; an analogue front-end (AFE) block 202 which performs an analogue signal processing including a gain adjustment, an analogue-to-digital (A/D) conversion, and the like with respect to image information; a digital signal processing block 203 which performs a predetermined image processing necessary with respect to the digitalized image data; a line memory 204 which stores the image data line by line; and a wireless module 205 which wirelessly outputs the image data toward a receiver and the like placed outside the subject.
Next, an example of imaging timings in a capsule endoscope provided with such an imaging apparatus is shown in FIG. 2. As shown in FIG. 2, the imaging timings are composed of: a timing of making a light source such as a light emitting diode (LED) emit light (i.e., a period for an exposure to the light); a period for reading from the image sensor 201; a period for data transmission; and blanking periods in the horizontal and the vertical directions. First by making the LED emit light during the vertical (V) blanking period of one frame period, a region to be imaged is exposed to the light and the region is captured by the image sensor 201 to obtain image information. The image information (analogue signal) which is captured and read by the image sensor 201 is digitalized as image data by undergoing a predetermined analogue signal processing in the AFE block 202, and once stored in the line memory 204 line by line after undergoing a necessary image processing in the digital signal processing block 203. Then, the image data stored in the line memory 204 is read and wirelessly output from the wireless module 205. Such operations are repeated for each line.
In addition, as shown in FIG. 3, there is another configuration which employs, instead of the line memory 204, a frame memory that can store image data for one frame and in which, after a charge accumulated in the image sensor 201 is reset, the LED is made to emit light to perform capturing by the image sensor 201, image data which is read out from the image sensor 201 and undergoes a predetermined analogue signal processing in the AFE block 202 is once stored in the frame memory frame by frame, and image data for one frame stored in the frame memory is read out to be wirelessly output from the wireless module 205.
In such an imaging apparatus, there is a case where a random noise is generated in the processing in the AFE block 202 which performs the analogue signal processing including the gain adjustment, the A/D conversion, and the like, and the noise gets on image data processed in the AFE block 202 and is wirelessly output as it is from the wireless module 205 to the outside of the body.
Such a random noise element can be balanced out and eliminated by capturing multiple images of a same frame object by the image sensor 201 through a repetition of the imaging timings shown in FIG. 2 for example, wirelessly outputting the image data processed in the AFE block 202 sequentially to the outside of the body, and performing, for example, an image averaging processing in the receiver which sequentially receives image data of these multiple images of the same frame object.
However, such a countermeasure causes deterioration in frame rate (in a case where image data of five images of the same frame object is, for example, used to perform the image averaging processing at a side of the receiver and treated as image data of one image, the frame rate deteriorates to one fifth), so that it becomes impossible to maintain a desired frame rate necessary as a capsule endoscope. Especially, since reading from the line memory which takes a substantial percentage of one frame period is repeated more than once, the frame rate easily deteriorates. To avoid the deterioration of the frame rate, it is only necessary to provide a plurality of line memories or frame memories to improve a processing speed. However, despite a demand for not increasing an amount of data to be wirelessly output and for making a size of a built-in circuit as small as possible from the view point of downsizing, low power consumption, and the like, a capsule endoscope, which is inserted into an inside of a subject and operates for a long time (eight hours, for example), goes against such a demand.